What are the difficulties in separating cast shadow from reflected light and shade?
I found that when I first starting looking I could see the main areas of shadow but missed many of the more subtle differences in tone caused by reflected light. The more that I looked then gradually the more able I was to understand better what I was seeing and pick more of these up. I found it easier to do this when I had one clear light source. I was surprised at just how much reflected light there is. Having a pale or reflective surface under the item being drawn resulted in much more reflected light as did any pale object like a wall or cupboard near to the still life. This then bounced light back into the scene as well.
The reflected shadow and light follows the contours of the objects. Have you shown this in your drawing?
I think that I have shown this in my drawing and to help me with this I tried to look as carefully as possible at the direction these took before I started to put them into the drawing so that I understood them as best I could.